One conventional type of nonvolatile memory cell is a SONOS device, which operates by trapping charge in a charge storage layer. The presence or absence of stored charge distinguishes a programmed cell from an unprogrammed cell.
Thus the ability of the cell to retain stored charge is crucial to its performance as a memory device. Stored charge tends to be lost from the cell over time and with successive write-erase cycles.
Attempts have been made to improve retention of charge by optimizing the characteristics of nitride (the material most frequently used for the charge storage layer), using oxynitride used instead of nitride, applying heat treatments to nitride, etc.
Some of these methods have shown some benefit, but there is a need to further improve retention and endurance of SONOS-type devices.